1. Field of the Invention
The invention relates to plating of materials and more particularly to vacuum plating of crystals, and to a method of plating crystals.
2. Description of the Prior Art
Crystals can be made to oscillate at a natural frequency, or by appropriate and selective plating can be maufactured to oscillate at a desired exact frequency. With the increased importance of electronics in our daily lives, crystals have gained widespread use in the circuitry for communications, and clocks and the like.
Depending upon how exacting the frequency is to be set the crystal may first be base plated, on one or both sides before being final coated, again on one or both sides. In the more accurate frequency work it is recommended that both sides of the crystal be coated to avoid "spurs" which is the occurrence of random peaks caused by aberrations of unknown origin.
In the frequency of the present invention the fundamental and overtones thereof have been tested up to 450 MHz and above, and have tested out with great accuracy of 2 ppm up to frequencies of at least 150 MHz.
In the prior art it was conventional practice to use a fixture carrying the crystal to be final plated and enclosed by a bell jar. Prior to plating the jar was evacuated. In this method the frequency accuracy was limited to 90 MHz, and even at this value test equipment showed the need to compensate to bring the frequency to within accepted levels.
Heretofore vacuum crystal plating was done in a bell jar in which the crystal had to be internally loaded with tweezers into a special holder within the bell jar. The entire bell jar then had to be evacuated, including the special holder for the crystal. The vacuum was then drawn thru a narrow opening and was not as absolute as desired. The relative size of the vacuum equipment, the complexly shaped items that were within the vacuum chamber provided multiple surfaces to which air or other contaminants could adhere. The presence of these contaminants, together with the difficulties in removing them resulted in deficiencies in the plating process and resultant deficiencies in the product produced.
Additionally, the internal loading of the crystals within the bell jar is relatively time consuming and therefore expensive. Also, because of the size and shape of the vacuum equipment, much of the noble metal vaporized in the plating process never reaches the crystals to be coated. Instead, it is randomly deposited on the walls of the bell jar or surfaces of the crystal holder. Therefore, the plating process is lengthened and much of the noble metal used in the plating process is ineffectively used.
The randomly deposited noble metal is recovered by cleaning the surfaces within the bell jar. However, because of the complexly shaped and relatively large surfaces the cleaning process is not as quick or easy as desired.